Figure 6. Conceptual model showing timing of in-channel sediment sampling for radionuclide analysis. In the model, samples were collected following the most recent timber harvest upstream of the study sites, at a time when impacts of elevated sediment delivery could be detected by 210Pb, but not 7Be. Details

Description

The type and intensity of land use in a watershed can considerably impact the form of channels within the basin. Of particular note with regard to land use are (A) the shifts in the amount of water reaching channels and how long that delivery of water takes to enter streams and (B) increased fine sediment (< 2 mm diameter) yield to channels that is associated with soil erosion in developed watersheds (for review see Walling 2006). This increased fine sediment yield is of particular concern to stream managers and ecologists, as large volumes of fines can have detrimental effects on the organisms which live in channels (Waters, 1995).

In particular, timber harvest can increase the amount of fine sediment reaching channels through the loss of soil cohesion that results from removing trees and their roots from the landscape. While fine sediment delivery following timber harvest has been studied with great detail in the steeplands of the Pacific Northwest, relatively little research has been completed in the eastern US, where streams and landscapes are often marked by lower gradients and the history of logging dates back over three centuries (Figure 1). This study focuses on such a watershed - the Narraguagus River in coastal Maine, where centuries of timber harvest may be negatively impacting habitat for endangered Atlantic Salmon (Salmo salar; NRC, 2004).

With regard to fish (salmon in particular), excessive levels of fine sediment can fill in the pore spaces between larger gravels and cobbles on the channel bed, a condition termed 'embeddedness'. Embeddedness results in the loss of important sheltering spaces for juvenile fish and other aquatic organisms. Because salmon lay their eggs in piles of gravel called 'redds,' large amounts of fine sediment clogging the pore spaces between these gravels can reduce the flow of oxygenated water to eggs, thus increasing mortality (Grieg, 2005). Additionally, fine sediments have been shown to adsorb pollutants in the water, presenting a toxic hazard to organisms dwelling in such channels.

There are many ways to quantify the amount of fine sediment that is present on channel beds. In this case, we (A) measured the median diameter (D50) of a number of grains on the bed (pebble count), (B) estimated the percent of the bed that was composed of fine sediment (an embeddedness survey), and (C) counted the number of spaces between large grains on the bed (a shelter space survey). While these techniques can tell us about the amount of fine sediment on the channel bed, radioisotope analyses can help us to understand the timing of fine sediment delivery to the stream. 210Pb and 7Be are two radionuclides produced in the atmosphere by cosmic rays interacting with atoms. These radionuclides are brought to earth in rain drops, and adsorb to exposed sediment on the landscape. We can detect sediment that has been "tagged" with 210Pb in the past ~110 years, and that which has been tagged with 7Be over the past ~265 days. Once these tagged sediments are brought to channels, they are no longer exposed to precipitation, and tagging ceases. Knowing this, we can measure how active channel-bed sediments are in 210Pb or 7Be, and thus make an estimate of how recently that sediment entered the channel (Matisoff et al., 2002; Whiting et al., 2005).

At 46 study sites along the Narraguagus River and selected tributaries in coastal Maine (Figure 2), we mapped the amount of area that had been logged since 1981 using three aerial photos (1981, 1996, 2007). We included areas that were within 1 km upstream of the study sites, and within 300 m of the channel (Figure 3). At each study site, we performed all of the sediment surveys described above: pebble counts, embeddedness surveys, and shelter space surveys (amount of fine sediment on channel bed), along with fine sediment 210Pb and 7Be samples (timing of fine sediment delivery to channel). Our findings indicate that the amount of upstream timber harvest since 1981 did not significantly influence the amount of fine sediment found on channel beds (Table 1). Further, 7Be was not significantly correlated with the amount of upstream logging since 1981 at our study sites. 210Pb, however, was significantly and positively correlated with the amount of upstream harvest (Table 1; Figure 4).

What do these findings mean with regard to the potential for recent timber harvest along the Narraguagus River to deliver fine sediment to the channel? It does not appear that an abnormally large quantity of fine sediment is found on channel beds which have seen a great deal of upstream logging since 1981, as pebble counts, embeddedness surveys, and shelter space surveys did not reveal any correlation. However, the results of analyses using 210Pb and 7Be reveal that fine sediment was preferentially delivered at sites with upstream timber harvest, but that this preferential delivery is no longer occurring (recall that 210Pb detects sediment delivered up to ~110 years ago, while 7Be can only detect deliver up to ~265 days prior to sampling). Thus, while sediment was likely delivered in increased quantities following recent (post-1981) timber harvest, much of that sediment has likely been flushed downstream. As a result, we believe that while logging along the Narraguagus River in Maine likely produces increases in fine sediment delivery immediately following harvest, that delivery is short lived and those sediments are quickly mobilized downstream. As a result, logging-driven fine sediment delivery does not appear to be a long-duration detriment to aquatic habitat in this watershed, perhaps due to present-day forest management and responsible harvest practices.